U.S. patent number 10,211,126 [Application Number 15/518,853] was granted by the patent office on 2019-02-19 for method of manufacturing an object with microchannels provided therethrough.
This patent grant is currently assigned to UNIVERSITY OF THE WITWATERSRAND, JOHANNESBURG. The grantee listed for this patent is UNIVERSITY OF THE WITWATERSRAND, JOHANNESBURG. Invention is credited to Ionel Botef, Agripa Hamweendo.
United States Patent |
10,211,126 |
Hamweendo , et al. |
February 19, 2019 |
Method of manufacturing an object with microchannels provided
therethrough
Abstract
This invention relates to a method of manufacturing an object
with microchannels provides therethrough, and more particularly,
but not exclusively, to a method of manufacturing a micro heat
exchanger with microchannels provided therethrough. The method
includes the steps of providing a metal base layer made from a
first metal; forming a plurality of spaced apart ridges, made from
a second metal, on the base layer; depositing more of the first
metal onto the ridges in order to cover the ridges; and removing
the ridges using a chemical etching process so as to produce
microchannels in a body made of the first metal.
Inventors: |
Hamweendo; Agripa (Lusaka,
ZM), Botef; Ionel (Johannesburg, ZA) |
Applicant: |
Name |
City |
State |
Country |
Type |
UNIVERSITY OF THE WITWATERSRAND, JOHANNESBURG |
Johannesburg |
N/A |
ZA |
|
|
Assignee: |
UNIVERSITY OF THE WITWATERSRAND,
JOHANNESBURG (Johannesburg, ZA)
|
Family
ID: |
55071076 |
Appl.
No.: |
15/518,853 |
Filed: |
October 13, 2015 |
PCT
Filed: |
October 13, 2015 |
PCT No.: |
PCT/IB2015/057821 |
371(c)(1),(2),(4) Date: |
April 13, 2017 |
PCT
Pub. No.: |
WO2016/059547 |
PCT
Pub. Date: |
April 21, 2016 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20170250122 A1 |
Aug 31, 2017 |
|
Foreign Application Priority Data
|
|
|
|
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Oct 14, 2014 [ZA] |
|
|
2014/07434 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23F
1/00 (20130101); B81C 1/00071 (20130101); F28F
3/12 (20130101); H01L 21/4878 (20130101); C23C
24/04 (20130101); H01L 23/46 (20130101); H01L
23/473 (20130101); C23F 1/02 (20130101); F28F
2260/02 (20130101); H01L 23/467 (20130101) |
Current International
Class: |
H01L
23/46 (20060101); H01L 21/48 (20060101); F28F
3/12 (20060101); C23F 1/00 (20060101); C23C
24/04 (20060101); B81C 1/00 (20060101); H01L
23/467 (20060101); C23F 1/02 (20060101); H01L
23/473 (20060101) |
Field of
Search: |
;216/39,40,41
;438/427,435,700,675,672 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Written Opinion of the International Searching Authority for
PCT/IB2015/057821. cited by applicant .
International Search Report for PCT/IB2015/057821. cited by
applicant .
Asgari, Omid, and Mohammad Hassan Saidi. "Approximate method of
determining the optimum cross section of microhannel heat sink,"
Journal of mechanical science and technology 23.12 (2009):
3448-3458. cited by applicant .
Gaikwad, V. P. "Microchannel heat sink fabrication techniques."
IOSR Journal of Mechanical and Civil Engineering (2009): 51-57.
cited by applicant .
Gargi, H.. Negi, V. S., Nidhi, and Lail, A. K, 2013, Numerical
Study of Microscale Heat Sinks Using Different Shapes & Fluids,
Central Scientific Instruments Organisation (CSIR-CSD), Excerpt
from the Proceedings of the 2013 COMSOL Conference in Bangalore,
India. cited by applicant .
Mihai, Ioan, "Heat transfer in minichannels and microchannels CPU
cooling systems." Heat Transfer-Theoretical Analysis, Experimental
Investigations and Industrial Systems, InTech, 2011. cited by
applicant .
Papyrin. A., Kosarev, V., klinkov, S., Alkhimov, A., and Fomin, V.,
2006, Cold Spray Technology, Summer Universities, ENISE,
St-Etienne, France. cited by applicant .
Prakash, Shashi, and Subrata Kumar. "Fabrication of microchannels:
A review." Proceedings of the Institution of Mechanical Engineers,
Part B: Journal of Engineering Manufacture 229.8 (2015): 1273-1288.
cited by applicant .
Upadhya, Girish, et al. "Micro-scale liquid cooling system for high
heat flux processor cooling applications." Semiconductor Thermal
Measurement and Management Symposium, 2006 IEEE Twenty-Second
Annual IEEE. IEEE, 2006. cited by applicant .
Zhao, C. Y., and T. J. Lu. "Analysis of microchannel heat sinks tor
electronics cooling." International Journal of Heat and Mass
Transfer 45.24 (2002): 4857-4869. cited by applicant .
Zhou, W., Deng, W., Lu, L., Zhang, J., Qin L., Ma, S., and Tang.
Y., 2014, Laser micro-milling of Microchannel on copper sheet as
catalyst support used in microreactor for hydrogen production,
International Journal of Hydrogen Energy, vol. 39, pp. 4884-4894,
ScienceDirect. cited by applicant.
|
Primary Examiner: Vinh; Lan
Attorney, Agent or Firm: Baker Donelson
Claims
The invention claimed is:
1. A method of manufacturing an object with microchannels provided
therethrough, the method comprising the steps of: providing a metal
base layer made from a first metal; forming a plurality of spaced
apart ridges, made from a second metal, on the base layer by
depositing the second metal onto the base layer by way of a cold
spraying process; depositing more of the first metal onto the
ridges in order to fill up a space between the ridges; removing
upper zones of the ridges, as well as the corresponding first metal
located between the ridges; depositing more of the first metal onto
the ridges in order to cover the ridges; and removing the ridges
using a chemical etching process so as to produce microchannels in
a body made of the first metal.
2. The method of claim 1, wherein the second metal is deposited
through a mask having parallel and spaced apart slots provided
therethrough, in order to form the plurality of spaced apart ridges
on the base layer.
3. The method of claim 1, wherein a sequence of depositing the
first metal and the second metal is repeated until a plurality of
layers of ridges have been formed before the ridges are removed by
way of chemical etching, so as to form a three dimensional network
of microchannels in the first metal.
4. The method according to claim 1, wherein the ridges are of
elongate configuration.
5. The method according to claim 1, wherein the ridges are parallel
relative to one another.
6. The method according to claim 1, wherein the metal base layer
comprises a solid metal substrate made from the first metal.
7. The method according to claim 1, wherein the metal base layer is
formed by depositing one or more layers of the first metal by way
of a surface coating process.
8. The method according to claim 1, wherein the second metal
comprises aluminum, zinc, copper, or any combination thereof.
9. The method according to claim 1, wherein the first metal
comprises copper and the second metal comprises aluminum.
10. The method of claim 1, wherein the upper zones of the ridges,
the first metal layer, or both, are removed by grinding.
11. The method of claim 1 further comprising a step of removing
upper zones of the ridges before the step of depositing more of the
first metal onto the ridges in order to cover the ridges.
12. The method of claim 11, wherein the upper zones of the ridges,
the first metal layer, or both, are removed by grinding.
13. The method of claim 1, wherein the ridges are substantially
triangular in cross-section.
14. The method of claim 13, wherein the ridges are substantially
trapezoidal in cross-section once the upper zones or apexes of the
ridges have been removed.
15. The method according to claim 1, wherein the first metal
comprises copper, gold, silver, nickel, aluminum or any combination
thereof.
16. The method according to claim 15, wherein the second metal
comprises aluminum, zinc, copper, or any combination thereof.
Description
This application is a National Stage Application under 35 U.S.C.
section 371 of PCT/IB2015/057821, filed Oct. 13, 2015, which claims
priority from and the benefit of ZA Application No: 2014/07434
filed Oct. 14, 2014, the entire contents of each application are
hereby incorporated by reference in their entireties.
BACKGROUND TO THE INVENTION
THIS invention relates to a method of manufacturing an object with
microchannels provides therethrough, and more particularly, but not
exclusively, to a method of manufacturing a micro heat exchanger
with microchannels provided therethrough.
Microchannels are channels provided in various devices having any
of the dimensions between 1 micron and 999 microns (Prakash and
Kumar 2014), i.e. channels with a hydraulic diameter of less than 1
mm. Microchannels are primarily used in advanced biomedical,
chemical, electronics, and mechanical engineering applications.
Depending on the applications, microchannels have different shapes,
sizes and structures and are fabricated from different substrate
materials that exhibit properties preferable for a particular
application.
For example, very fast and reliable compact computers require
microprocessors with very high clock speed. However, the impediment
of these advanced microprocessors is that they emit large heat flux
densities which require novel cooling technologies in order to keep
the temperature of the electronic components below critical levels
(Zhao et al. 2002, Upadhya et al. 2006). At present, the heat flux
densities emitted by microprocessors exceed the capability limit of
the existing air cooling technologies that ultimately compromises
the performance and reliability of the computers due to increased
operation temperatures (Mihai 2011).
To address this problem, microfluidic cooling systems have been
proposed as innovative thermal solutions for cooling the
contemporary and next generation high speed computer
microprocessors. Microfluidic cooling systems exhibit superior
thermal extraction capabilities compared to other cooling
technologies. In this regard, FIG. 1 presents the hierarchy of
thermal extraction mechanisms performance, and shows how
microfluidic cooling systems outperforms the other heat extraction
systems.
A typical microfluidic cooling system is shown in FIG. 2 and
comprises a micro heat sink, a micro pump, a micro condenser, and a
fan. All these components operate on a closed loop micro-scale
principle (Upadhya et al. 2006). During operation, the micro pump
pumps the cooling liquid from the micro condenser to the micro heat
sink, where the cooling liquid traverses the micro heat sink via
the microchannels acting as heat transfer conduits. It is in these
microchannels where the heat, which is conducted from the heat
spreaders of the microprocessor, is transferred to the cooling
liquid which in turn absorbs heat. The heated liquid then removes
the heat to the micro condenser, where dissipation to the
atmosphere occurs with the assistance of air cooling of the fan.
The coordinated operation of these components accounts for the
superior thermal performance of the microfluidic cooling systems
(Upadhya et al. 2006).
Depending on the applications of microchannel-based devices,
different types of materials are preferred. Polymeric and glass
substrates are mostly used in biomedical and chemical devices,
while silicon-based substrates and metallic substrates are used for
electronics and mechanical engineering-related applications
(Prakash and Kumar 2014). In recent years, polymeric substrate
microfluidic devices started to exceed the use of silicon and glass
substrates, mainly because of their low production costs and their
high chemical resistance to an operating environment. Also,
metallic microchannels have gained considerable attention as
cooling devices in electronic and mechanical applications because
many endothermic and exothermic reactions can be performed on such
metallic substrates. The metallic microchannels can furthermore
withstand corrosive environments, and can reach operating
temperatures as high as 650.degree. C. (Prakash and Kumar
2014).
A further important consideration is that shapes, sizes and
structures of microchannels vary depending on the particular
application for which it is to be used. Most common cross sections
include rectangular microchannels, square microchannels, circular
microchannels, half circular microchannels, U-shape microchannels
and Gaussian beam shape microchannels (Prakash and Kumar 2014).
Also, whilst most of the microfluidic channels have high
area-to-volume aspect ratios, low area-to-volume aspect ratio
channels are also not uncommon in applications such as particle
separation devices (Prakash and Kumar 2014).
Studies have shown that different microchannels with different
cross sections exhibits different heat removal performance, with
the trapezoidal shape microchannels outperforming the other
possible microchannels' shapes in terms of heat extraction
capability (Asgari et al.). Also, this performance varies with the
geometric dimensions of the microchannels (Gargi et al. 2013) which
are also influenced by the technology used to fabricate those
microchannels (Zhou et al. 2014). There is, however, a scarcity of
micro heat sinks with trapezoidal channels due to the lack of
robust microfabrication methods (Upadhya et al. 2006, Gargi et al.
2013).
The large scale fabrication of microchannels in typical substrates
has always been a difficult task because of: (1) the precision
required in the manufacturing of such products; and (2) the lack of
suitable technologies to fabricate these devices (Prakash and Kumar
2014).
The methods used for fabricating different types of microchannels
include both conventional and nonconventional fabrication
techniques. However, the contemporary microfabrication technologies
for microchannels could be broadly categorized as additive or
subtractive, depending on whether the material is added or
subtracted during the microfabrication process, as summarized in
FIG. 3. The main groups of these technologies are:
stereolithography, chemical etching; and micro-machining.
The most common fabrication processes for microchannels are
discussed in more detail below.
Micro-machining of microchannels is particularly suitable for the
fabrication of individual personalized components rather than
fabrication of large batch sizes. This group of methodologies
evolved as a result of the advent of ultra-precision machining
tools that can achieve high level of machining accuracy at high
machining speeds, whilst also resulting in good surface finish on a
large number of materials, such as steel, aluminum, brass, or
plastics and polymers (Prakash and Kumar 2014). Micro-machining is
the most diverse category of microfabrication technologies and is
composed of advanced micro milling, laser cutting, and electrical
discharge machining (EDM) or a combination of these processes.
These processes do not require a very expensive setup, which
enables them to be used to produce micro-devices in small quantity
and at a reasonable cost (Prakash and Kumar 2014).
Advanced micro milling has a drawback of limited tool geometries,
which makes it difficult to fabricate microchannels with sizes
below 500 .mu.m. Laser technology, which uses a collimated laser
beam to groove substrates, is a time consuming process which is not
suitable for mass production and the technology cannot fabricate
preset geometric dimensions due to laser interaction with the
materials. The EDM method has the challenge of low rate of material
removal and therefore is also not well suited for mass production
of micro heat sinks (Zhou et al., 2014). A common drawback of all
the micro-machining processes is therefore that it is not suitable
for the high volume mass production of microchannel devices.
Chemical etching, the most widely used subtractive technique for
micromachining, could be described as pattern transfer by chemical
or physical removal of material from a substrate, often in a
pattern defined by a protective mask layer such as a resist or an
oxide (Prakash and Kumar 2014). Chemical etching could be wet or
dry. In dry etching, mostly utilized for glass and polymer base
materials, the surface can be physically etched in the gas or
vapour phase by ion bombardment, can be etched by a chemical
reaction at the surface, or can be etched by combining the physical
and chemical mechanisms. Wet etching is suitable for metallic
substrates that react well with chemicals, but the process results
in non-parallel walls on the glass surface and, as the channel
etches deeper, the walls are also etched (Prakash and Kumar 2014).
In addition, chemical etching method has very low productivity and
the process does not lend itself to precise control of the
geometric dimensions of the fabricated microchannels.
Lithography is one of the major fabrication techniques used to
fabricate microchannels. This process enables the fabrication of
many different types of topographies that are difficult to generate
using other fabrication techniques (Prakash and Kumar 2014). The
most widely used form of lithography is where pattern transfer from
mask onto thin films is done by photolithography. In recent times,
X-ray lithography has also been used to create polymer
microchannels that, in contrast with ion-beam lithography and
electron beam lithography, do not require the presence of vacuum
and clean room facilities, which makes this process cheaper and
faster (Prakash and Kumar 2014). Furthermore, LIGA--the German
abbreviation for Lithography, Galvanoformung (electroplating) and
Abformung (Molding)) enables the precise manufacturing of high
aspect ratio microchannels ranging from 100 to 1000 microns, and
enables the use of new building materials and the fabrication of a
wider dynamic range of dimensions and shapes (Prakash and Kumar
2014). However, its applicability is restricted by high costs, as
well as the production of toxic waste (Zhou et al. 2014).
The above limitations of contemporary microfabrication technologies
provide overwhelming evidence that, up to now, there has been no
robust method for the mass microfabrication of microchannels with
trapezoidal cross-sectional profiles. Therefore, new methods for
faster and cheaper production of these devices must be explored for
sustainable development in this area, in particular since there is
a growing demand for microchannels with trapezoidal cross-section
for use in micro heat exchangers.
It is accordingly an object of the invention to provide a method of
manufacturing an object with microchannels provides therethrough
that will, at least partially, alleviate the above
disadvantages.
It is also an object of the invention to provide a method of
manufacturing an object with microchannels provides therethrough,
which will be a useful alternative to existing methods.
SUMMARY OF THE INVENTION
According to the invention there is provided a method of
manufacturing an object with microchannels provided therethrough,
the method including the steps of: providing a metal base layer
made from a first metal; forming a plurality of spaced apart
ridges, made from a second metal, on the base layer; depositing
more of the first metal onto the ridges in order to cover the
ridges; and removing the ridges using a chemical etching process so
as to produce microchannels in a body made of the first metal.
There is provided for the ridges to be formed by depositing the
second metal onto the base layer by way of a surface coating
process.
Preferably, the second metal is deposited through a mask having
parallel and spaced apart slots provided therethrough, in order to
form the plurality of spaced apart ridges on the base layer.
There is provided for the sequence of depositing the first metal
and the second metal to be repeated until a plurality of layers of
ridges have been formed, before the ridges are removed by way of
chemical etching so as to form a three dimensional network of
microchannels in the first metal matrix.
A further intermediate step provides for the upper zones of the
ridges to be removed before the step of depositing more of the
first metal onto the ridges in order to cover the ridges.
Alternatively, the space between the ridges may first be filled by
the first metal, following which the upper zones of the ridges, as
well as the corresponding first metal located between the ridges,
are then removed before the step of depositing more of the first
metal onto the ridges in order to cover the ridges.
The upper zones of the ridges and/or the first metal layer may be
removed by grinding, or any other suitable machining process.
There is provided for the ridges to be substantially triangular in
cross-section.
There is further provided for the ridges to be substantially
trapezoidal in cross-section once the upper zones or apexes of the
ridges have been removed.
There is provided for the ridges to be of elongate
configuration.
There is provided for the ridges to be parallel relative to one
another.
The metal base layer may be in the form of a solid metal substrate
made from the first metal.
Alternatively, the metal base layer may be formed by depositing one
or more layers of the first metal by way of a surface coating
process.
In a preferred embodiment the surface coating process will be a
cold spraying process.
There is provided for the first metal to be selected from the group
including copper, gold, silver, nickel, aluminium or any
combination thereof.
There is provided for the second metal to be selected from the
group including aluminium, zinc or copper.
In one embodiment of the invention the first metal is copper, and
the second metal is aluminium.
In another embodiment of the invention the first metal is a
copper/gold metal mixture, and the second metal is aluminium.
In a further embodiment of the invention the first metal is a
copper/gold metal mixture, and the second metal is zinc.
In a further embodiment of the invention the first metal is silver,
and the second metal is copper.
In a further embodiment of the invention the first metal is silver,
and the second metal is aluminium.
In a further embodiment of the invention the first metal is nickel,
and the second metal is zinc.
In a further embodiment of the invention the first metal is nickel,
and the second metal is aluminium.
In a further embodiment of the invention the first metal is nickel,
and the second metal is aluminium.
In a further embodiment of the invention the first metal is copper,
and the second metal is zinc.
In a further embodiment of the invention the first metal is
aluminium, and the second metal is zinc.
In a further embodiment of the invention the first metal is
aluminium, and the second metal is copper.
There is provided for the manufactured object to be a micro heat
exchanger.
It should be noted that, for the purposes of this specification,
the term `metal` may refer to single metal, or to a combination of
metals.
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the invention is described by way of a
non-limiting example, and with reference to the accompanying
drawings in which:
FIG. 1 is a representation of the thermal efficiency of a plurality
of heat transfer mechanisms known in the art;
FIG. 2 is a schematic illustration of a micro-fluidic cooling
system as is known in the art, and which incorporates a micro heat
exchangers having microchannels provided therethrough;
FIG. 3 provides a breakdown of microfabrication technologies known
in the art;
FIG. 4 shows the setup of a cold spray process used in the method
of this invention, and in particular the cold spray process as
configured to deposit a first metal used in the method;
FIG. 5 shows the setup of the cold spray process used in the method
of this invention, and in particular the cold spray process as
configured to deposit a second metal used in the method;
FIG. 6 shows the steps of the method in accordance with the
invention, utilizing the cold spray setups of FIGS. 4 and 5;
FIG. 7a is a photograph of a cross-section through the manufactured
object in an intermediate state, before the second metal has been
removed from the object;
FIG. 7b is a photograph of a cross-section through the manufactured
object in a final state, once the second metal has been removed
from the object, and in which trapezoidal microchannels are
visible;
FIG. 8a shows the geometry of the heat sink with microchannels
provided therethough based on the manufactured heat sink of FIG.
7b;
FIG. 8b shows a computational model of the heat sink of FIG. 8a;
and
FIG. 9 depicts the temperature distribution through the heat sink
achieved by way of a computational simulation.
DETAILED DESCRIPTION OF INVENTION
Before any embodiments of the invention are explained in detail, it
is to be understood that the invention is not limited in its
application to the details of construction and the arrangement of
components set forth in the following description or illustrated in
the following drawings. The invention is capable of other
embodiments and of being practiced or of being carried out in
various ways. Also, it is to be understood that the phraseology and
terminology used herein is for the purpose of description and
should not be regarded as limiting. The use of "including,"
"comprising," or "having" and variations thereof herein is meant to
encompass the items listed thereafter and equivalents thereof as
well as additional items. The terminology includes the words
specifically mentioned above, derivatives thereof, and words or
similar import. It is noted that, as used in this specification and
the appended claims, the singular forms "a," "an," and "the," and
any singular use of any word, include plural referents unless
expressly and unequivocally limited to one referent. As used
herein, the term "include" and its grammatical variants are
intended to be non-limiting, such that recitation of items in a
list is not to the exclusion of other like items that can be
substituted or added to the listed items.
Referring to the drawings, in which like numerals indicate like
features, a non-limiting example of a method of manufacturing an
object provided with microchannels therethrough in accordance with
one embodiment of the invention is generally indicated by reference
numeral 10.
In this particular embodiment, a cold spray technique is used as a
material deposition methodology. Cold spraying is one of the most
recent surface coating innovations through which it is possible to
coat several metals by exposing a metallic or dielectric substrate
to a high velocity (300-1200 m/s) jet of small (5-50 .mu.m)
particles accelerated by a supersonic stream of compressed and
preheated gas (Papyrin et al.).
During the cold spraying process, the gas temperature is always
lower than the melting point of the particle's material, so, upon
impact on the substrate, these high-velocity `cold` particles
deform plastically and bond with the underlying material resulting
in coating formation of the particles in their solid state. The
deposition process takes place at temperature far below the melting
point of the metal powders, and cold spraying is, therefore, a
solid state deposition process that provides distinctive advantages
when compared to the traditional thermal spraying processes. These
advantages include (Papyrin et al.): high thermal/electrical
conductivity of coatings; minimal oxidation and undesirable phases
changes in coatings; retention of initial materials properties; low
thermally induced residual stresses; capability to spray thermally
sensitive materials; capability of coating highly dissimilar
materials' combinations; needs minimum substrate surface
preparation; high productivity due to high powder feed rate,
automation and process adjustability; and a cost effective process
due to high deposition efficiencies for many materials. As a result
of these advantages, cold spraying has been selected to be used for
the development of a novel method for the fabrication of
microchannels.
Two cold spraying configurations are utilised during the method in
accordance with the invention, and are shown in FIGS. 4 and 5.
FIG. 4 shows a cold spraying setup 20 where a gun 21 ejects a jet
of a first metal powder 23, which is then deposited onto a
substrate 22. The distance between the gun 21 and the substrate
(SD.sub.GS) is referred to as the standoff distance between the gun
21 and the substrate 22. In this setup no mask is provided between
the gun 21 and the substrate 22.
FIG. 5 shows a further cold spraying setup where a gun 31 ejects a
jet of a second metal powder 33, which is then deposited onto a
substrate 32. However, in this case the jet 33 passes through a
mask 34 having predetermined openings (in the form of spaced apart,
parallel, elongate slots) provided therethrough, and the coating on
the substrate 32 is therefore not uniform. Instead, a plurality of
discrete ridges 35, or hillocks, are formed on the substrate 32. In
this setup, the distance between the gun 31 and the mask 34
(SD.sub.GM) is referred to as the standoff distance between the gun
31 and the mask 34, and the distance between the mask 34 and the
substrate 32 (SD.sub.MS) is referred to as the standoff distance
between the mask 34 and the substrate 32. These two cold spraying
setups are used in combination during the method that will now be
described in more detail below.
Generically, the method (depicted by sequence 10 in FIG. 6)
comprises a number of iterative sequences, and commences by the
deposition of a first material 23 (for example copper) to form a
first substrate using the setup 20 described with reference to FIG.
4. Next, a second material 33 (for example aluminium) is deposited
onto the first substrate 22 using the setup 30 described with
reference to FIG. 5. In this step the material 33 is deposited
through a mask 34, resulting in the formation of a plurality of
spaced apart and discrete ridges or hillocks 35 formed on the first
substrate 22. These ridges or hillocks 35 are generally triangular
when viewed in cross-section. Thereafter, a further layer(s) of the
first material 23 is deposited onto the workpiece to a level where
it covers at least part of the ridges or hillocks 35. It is
envisaged that the further layer may even cover the ridges or
hillocks 35 in totality. During a subsequent grinding step 40, a
grinding tool 41 is used to grind the protruding tips 35.1 of the
ridges or hillocks 35 to a predetermined height, in order for the
new upper edge 35.2 of the ridges 35 to define upper ends of now
trapezoidal shaped ridges or hillocks 35. A further layer of the
first material is subsequently deposited onto the work piece, and
the sequence is repeated until a desired number of trapezoidal rows
of ridges or hillocks 35 have been formed.
Once the above process has been completed, the second material is
de-alloyed and as such removed from the work piece using a suitable
chemical process. For example, if the base material (first
material) is copper, and the intermediate material (second
material) is aluminium, the aluminium can be removed using 30%
diluted hydrochloric acid. In this way a copper body is formed,
having a plurality of trapezoidally cross-sectioned microchannels
provided therethrough.
SPECIFIC EXAMPLE
A specific, but non-limiting, example of how the above method was
put into practice is now described in more detail below.
In order to develop the new method for microfabrication of
microchannels, the following tasks were carried out: suitable metal
powders were selected; cold spraying process parameters were
optimized; alloying was done using cold spraying processes (as
shown in FIGS. 4 and 5); de-alloying was effected in order to
fabricate the microchannels; and products were characterised.
The following materials and equipment were used: metal powders
(Copper matrix former as a first metal; Aluminium as a second
metal; a microchannels forming agent; and Alumina grit blast for
activation of substrate surface); consumable materials: distilled
water, and dilute hydrochloric acid (30%) for de-alloying; and
equipment (cold spraying equipment from Centerline, Canada; slotted
mask; surface grinder; and an optical microscope).
The optimisation of the cold spraying fabrication process was
conducted to establish optimal spray parameters. The process
parameters considered during laboratory experiment included: gas
temperature (T.sub.0); gas pressure (P.sub.0); standoff distance
between gun and mask (SDGM); standoff distance between the
substrate and mask (SDSM); standoff distance between gun and
substrate (SDGS); traversing speed of the gun (V); and powder feed
rate (F).
During these trials, aluminium (Al) powder was sprayed through the
mask onto the activated copper (Cu) substrate to fabricate the Al
ridges. The optimised parameters from this process were recorded
when the width of the fabricated Al ridges were less or equal to
120% the width of the slots of the mask. Also, at this point, it
was ensured that the geometric profile of the ridges was consistent
and that there was no overspray of Al powder.
Furthermore, during optimisation of Cu deposition, Cu was sprayed
without the mask, and the optimised parameters were recorded when
rapid build-up of Cu coating occurred and the colour of the coating
did not change to brown. During copper cold spray coatings, the
coating surface present different colors attributed to the
different surface temperatures of coatings deposited at different
standoff distances. A relatively high temperature causes the
oxidation and this could be observed at a shorter standoff
distance. In this example, because the colour didn't change to
brown, it indicates a good coating.
In all these trials, single factor variation of process parameters
was implemented. The optimal CS process parameters are shown in
Table 1.
TABLE-US-00001 TABLE 1 Optimal process parameters for Al and Cu.
T.sub.0 P.sub.0 SD.sub.GM SD.sub.SM SD.sub.GS V F Powder (.degree.
C.) (bars) (mm) (mm) (mm) (mm/s) (%) Al 390 8 25 2 -- 10 5 Cu 400
9.5 -- -- 10 10 40
The next step in the new method consists in the microfabrication of
microchannels using the alloy-de-alloy concept. In this process, a
cold spraying process was used to alloy Al into a Cu matrix and
de-alloying was accomplished by etching specimens in acid to
selectively dissolve the Al. Initially, the Cu substrate was
activated by grit blasting using Alumina powder and spray
parameters for Al as shown in Table 1. To alloy, the spray
parameters given in Table 1 were used to alternately deposit Al and
Cu according to the process sequence illustrated in FIG. 6.
The alloying steps consisted of the following steps: 1. Spray three
layers Cu powder; 2. Through the slotted mask, spray one layer of
Al powder to fabricate the Al ridges; 3. Without the mask, spray
three layers of Cu to burry bottom potion of the ridges. This
determines the depth of the trapezoidal microchannels; 4. Grind off
the protruding tips of the Al coatings to make the trapezoidal
ridges; 5. Repeat step 1 to 4 until there are three layers of Al
ridges; 6. Without the mask, spray three layers of Cu to cover the
top layer of Al ridges; and 7. Lightly grind the periphery of the
specimen to expose the cross sections of the Al ridges.
The de-alloying steps consisted of the following steps: 8. Immerse
the specimen in 30% dilute hydrochloric acid to dissolve the Al
ridges; 9. Repeat step 8 until there are no more bubbles forming
around the specimen; and 10. rinse the specimen in distilled
water.
The process was followed by the analysis of the microfabricated
channels. The microfabricated specimens were sectioned,
metallographically polished and then characterised for morphology
by taking images using the Optical Microscope (OM). The geometric
dimensions of the microchannels were measured using the OM. Ten
measurements each side were taken and averaged. These measurements
were used to computationally model the microchannels to mimic the
micro heat sink. To evaluate the cooling capability of the modelled
micro heat sink, de-ionised water was chosen as the cooling fluid
flowing through the microchannel. Flow was assumed to be laminar
and the forced convection heat transfer coefficient which was
calculated under these conditions was 7,246 W/m.sup.2K. Heat flux
densities of 100, 200, 300, 400, and 500 W/m.sup.2 which represent
the heat fluxes emitted by the microprocessors to the micro heat
sink were applied. The ambient temperature was taken to be
20.degree. C.
Based on process route presented above, the Al ridges were alloyed
inside the Cu matrix and the insertion before de-alloying is shown
in FIG. 7a. After de-alloying, a three-layered porous Cu
microchannel with trapezoidal sections was fabricated, and their
cross section is shown in FIG. 7b. The average dimensions of each
side of these microchannels were acquired and are shown in Table 2.
These measurements related to the geometric structures of the
modelled micro heat sinks are shown in FIG. 8a. FIG. 8b shows the
computational model of the Cu micro heat sink. Furthermore, the
simulated variation of heat flux with junction and surface
temperature of the Cu micro heat sink are shown in Table 3, and
FIG. 9 depicts the temperature profile on the surface of the micro
heat when the maximum heat flux of 500 W/m.sup.2 is applied to the
micro heat sink by the microprocessor.
TABLE-US-00002 TABLE 2 Geometric dimensions for fabricated
microchannels (relate to FIG. 8a) Dimension a b c d e h w l t
t.sub.w .mu.m 1091 443 300 400 650 436 4 132 7500 2708 870
TABLE-US-00003 TABLE 3 Variation of heat flux with temperature of
the Cu-- micro heat sink Heat Flux Q W/cm.sup.2 100 200 300 400 500
Junction Temp (.degree. C.) 22 22 25 27 28 Surface Temp (.degree.
C.) 20 20 20 20 20
In addition to the examples given above, there are several other
combinations of metals that could be used in this respect including
gold, silver and nickel. Nickel has already been sprayed combined
with other metals, while silver can also be sprayed. From these
results, it is probable that other metals such as gold can be
sprayed when combined with other metals such as copper or nickel as
binders. Spraying of metal mixtures is also possible. Further,
besides aluminium, several other metals can be preferentially
etched out of the matrix or their combinations of metals as
illustrated in the following pairs:
TABLE-US-00004 Second (removed) First (base) material material
De-alloying chemical Gold & Copper Aluminium Sulfide solutions
Gold & Copper Zinc NaOH, Na2SO4, and salty NaCl Silver Copper
Hydrofluoric acids Silver Aluminium Hydrochloric acid Nickel Zinc
Dilute Nitric Acid Nickel Aluminium Sulphuric Acid Copper Aluminium
Hydrochloric acid Copper Zinc Hydrochloric acid Aluminium Zinc
Hydrochloric acid
Currently available commercial techniques, such as
stereolithography, selective laser sintering, or fused deposition
manufacturing, are able only to produce prototypes using wax,
plastic, nylon, paper, polycarbonate materials, etc. However,
material melting and solidification created difficulties that have
hindered the widespread adoption of these techniques. There are
many difficulties that must be addressed when attempting to use
these techniques with materials with high melting temperature such
as metal. Also, their applicability is restricted by high costs, as
well as the production of toxic waste. The chemical etching method
has very low productivity and the process does not lend itself to
precise control of the geometric dimensions of the fabricated
microchannels. Micro-machining of microchannels is particularly
suitable for low volume production and has drawbacks such as
limited tool geometries which make it difficult to fabricate
microchannels with sizes below 500 microns. Consequently, the newly
developed method for fabrication of microchannels has different
process routes to those of stereolithography, chemical etching and
micro-machining processes. In addition, Cu/Al is one of the
preferred embodiments due to the fact that Copper is ideally suited
to cold spraying and the resulting coating possesses excellent
electrical and thermal conductivity.
The new method is a hybridisation of additive and subtractive
microfabrication and so this new method could be added as a new
group of microfabrication technological process.
The simulation of the fabricated microchannels indicate very high
heat transfer capability since they can keep the mean temperature
of the microprocessor at 4.degree. C. above ambient temperature,
and which it is far below the critical temperatures of
55-100.degree. C. required for the commercially available
microprocessors (Mihai 2011). The cooling capability is also in
line with the microfluidic cooling systems that have superior
thermal extraction capability compared to any other thermal
solution (Upadhya et al. 2006).
In addition, and unlike other microfabrication technologies
presented in the published literature, this new method for the
fabrication of microchannels offers: repeatability of the geometric
profile requirements for specific microchannels' design; high
production flexibility since the process parameters could be
independently altered which result in microchannels with different
geometric dimensions; and a very short throughput time, thereby
making it a very strong candidate for mass production of micro heat
sinks.
It will be appreciated that the above is only one embodiment of the
invention and that there may be many variations without departing
from the spirit and/or the scope of the invention.
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